The present invention relates to a QCM (Quartz Crystal Microbalance) sensor for detecting and estimating (quantitatively analyzing) components of a sample from either a variation in an oscillation frequency of a quartz oscillator or a variation in an impedance when a surface of an electrode of the quartz oscillator is immersed into a sample gas or a sample solution, and particularly relates to a multi-channel QCM sensor device suitable for detecting and estimating simultaneously a plurality of components from the same sample.
In chemical and bio-chemical fields, it is important to quantitatively analyze a reaction quantity and a mass of a product. However, it has been difficult to obtain a sufficient detection sensitivity to an extremely minute quantity of reaction.
In chemical and bio-chemical sensors to which a microbalance principle is applied using a crystal oscillator of AT-cut have been developed and have been noticed to the public.
The crystal oscillator of AT-cut has its main resonant frequency inversely proportional to a thickness of a disk of the oscillator. In this case, when the components of the sample are film formed on surfaces of electrodes of the crystal oscillator or an adsorption of a substance occurs on the surface of the electrodes, a frequency shift phenomenon occurs which corresponds to a weight of the substance present on the surface per unit flat surface area.
The QCM sensor is an application of the above-described frequency shift phenomenon. Since the crystal oscillator of AT-cut has a frequency characteristic which is stable in a wide range of temperature, a stable detection sensitivity can be expected. If a condition is established, it is possible for the QCM sensor to detect an adsorption substance of 1 through 10 ng on a real time basis.
A relationship between the substance quantity of the adsorption and the shift quantity of the frequency will be described below.
First, the oscillation frequency of the crystal oscillator of AT-cut is expressed in an equation (1) and an equation (2) shown in FIG. 12.
In each equation (1) and (2), fo: a main resonant frequency of the crystal oscillator, xcexd: a sound velocity in the crystal, tq: a thickness of the crystal, xcexcq: a shear elasticity constant, and xcfx81q: a density of the crystal.
A mass change xcex94m generated on the surface of the crystal oscillator having the main resonant frequency fo is expressed in an equation (3) of FIG. 13 by evolving the relation equation between the main resonant frequency and thickness of the crystal.
In the equation (3), xcex94f denotes a frequency variation due to an addition of a mass, Apiezo denotes an electrical effective area, Cf denotes a whole sensitivity.
If the crystal oscillator having the main resonant frequency fo of the equation (3) be rewritten as in the equation (4) shown in FIG. 14 since xcex94f is affected by the viscosity of the liquid and density thereof.
It is noted that xcex7L denotes a viscosity of the solution, xcfx81L denotes a density of the solution, and xcfx890=2 xcfx80f0.
In addition, the whole sensitivity Cf is expressed in the equation (5) of FIG. 15.
As appreciated from the above equation of (5), it is important to increase the main resonant frequency fo to increase the whole sensitivity Cf. Since the whole sensitivity Cf itself is a function of the frequency, a deviation xcex94f of the frequency is actually dependent upon a 3/2 power of the main resonant frequency fo.
Hence, as the main resonant frequency of the crystal oscillator used as the sensor is increased, a high sensitivity sensor can be used. For example, FIG. 16 shows a characteristic graph plotting the frequency shift quantity xcex94f of the crystal oscillator immersed into a glucose solution of 15 wt % (weight percents). As the main resonant frequency fo becomes increased, it will be appreciated that the deviation of the resonant frequency with respect to the adsorption quantity on the same electrode surface.
As described above, the crystal oscillator of AT-cut uses a thickness-slip mode, the main resonant frequency fo is inversely proportional to its thickness tq. In addition, it is necessary for the crystal oscillator to reduce its electrode effective area in proportion to the frequency in order to obtain a sufficient value of xcex3 (xcex3 denotes a ratio between a parallel capacitance and a serial capacitance in an equivalent circuit of the crystal oscillator and usually approximately 250 in the case of the crystal oscillator of AT-cut).
For the reason described above, it becomes necessary to provide the crystal oscillator having a small electrode area and a thin crystal thickness for that used for a high-frequency application purpose.
On the other hand, in order to realize the QCM sensor, there is an arrangement of the QCM sensor in which the crystal oscillator 1 is held within a vessel 2, only the oscillator surface is exposed to be immersed into the sample, its surrounding of the surface is sealed by means of O ring or so on, and electrodes 1A and 1B of the crystal oscillator 1 are connected across an oscillation circuit or an impedance measurement circuit 4 via lead wires, as shown in FIG. 17.
Since the QCM sensor arranged as described above has a crystal substrate which is thin in accordance with the high frequency purpose crystal oscillator, the substrate often becomes distorted (have a strain) or cracked due to a stress imposed on its sealing portion.
Hence, it is difficult to put the sensor device for the high frequency application into practice. However, a method of making the QCM sensor device through a method of making only a center portion of the substrate at a single cell thinner by means of an etching process has been proposed by Zuxuan Lin et al.
In this case, a portion of the crystal oscillator corresponding to a frame of the crystal oscillator has a thickness corresponding to 5 through 6 MHz (corresponding to 0.3 mm) conventionally used and a large distortion of that portion due to the sealing does not occur. In addition, since the portion which has been thinned has the sufficiently small-sized electrode area to provide an energy trap, an effect of the frame is difficult to receive.
Although the QCM sensor to increase its sensitivity can be achieved by the above-described method, any of the conventional QCM sensors is arranged in such a manner that only one sensor is within a single cell. Hence, any conventional QCM sensor can only measure one a single component from one sample.
This makes the measurement of the respective components of the sample restricted to one-cell-for-one-sample with the cells capable of detecting and measuring the respective cells in order to detect and estimate the respective components from the sample solution including the plurality of components. Consequently, it takes a long time to measure the individual components and a measurement cost becomes increased.
In order to shorten the measurement time, a multi-channel type QCM sensor has been proposed. Such a multi-channel type QCM sensor as described above is arranged as follows: a plurality of crystal oscillators are attached onto a substrate holder, a probe is moved on each crystal oscillator, and a data on each component of the sample is obtained for each crystal oscillator.
However, in the multi-channel type QCM sensor described above, the application of an electrical field is caused by the movement of the probe.
A deviation of the relative position of the probe to each crystal oscillator causes the oscillation frequency and the impedance to be varied.
The conventional multi-channel QCM sensor is difficult to be actually arranged to maintain accurately a measurement condition such as the resonant frequencies of the crystal oscillators.
In addition, a consequent stable measurement cannot be desirably be achieved by the conventional QCM sensor.
It is therefore an object of the present invention to provide a QCM sensor which enables a stable measurement of each component of a sample with a sensor portion being in a multi-channel structure and enables a highly accurate measurement thereof with a fundamental resonant frequency of a sensor portion increased to a high frequency point.
In addition, the sensor device according to the present invention has a structure such that a rate L/t between a thickness t of the crystal substrate and a distance L between the adjacent electrodes is equal to or above 20 if the electrodes of the multi-channel structures are circular.
According to the structure described above, a highly accurate measurement without mutual interference between the adjacent electrodes can be made in order to once detect and estimate the components different for the electrodes by one sample and a whole dimension of the sensor device can be minimized.
The sensor device according to the present invention is a multi-channel structure in which the electrodes are adjoined and disposed at a plurality of portions, a receptor different for each component of the sample to be detected and to be estimated being fixed onto each electrode. This structure can detect and estimate once the components different for each electrode by one sample. In addition, since it becomes unnecessary to operate the probe to be moved on the conventional multi-channel type QCM sensor, the stable measurement can be made without variation in the measurement condition.
Furthermore, the structure becomes simple since the device structure is only the sensor device and the measurement device.
The crystal substrate of the sensor device according to the present invention is provided with a separation groove to reduce an oscillation energy between the adjacent electrodes. This structure causes a leakage of the oscillation energy between the electrodes to be attenuated into the separation groove and permits a stable measurement frequency with the distance between the electrodes shortened.
The crystal substrate of the sensor device according to the present invention has the structure such that a thickness of an electrode forming portion becomes thinner than that of the surrounding portion.
The crystal substrate of the sensor device according to the present invention can enhance a mechanical strength of the substrate to secure its holding and can use it in the high frequency range with the thickness of the electrode portion thinned.
The sensor device according to the present invention includes a sensor main body in which the electrode forming portion using a high-frequency thin crystal substrate is made thinner than the thickness of the surrounding portion and a substrate holder made of a crystal substrate or a quartz substrate, and whose thickness is larger than the sensor device main body, and onto which the sensor device main body is adhered.
According to this structure, a quantity of etching to make the electrode portion thinner in order to make a high frequency sensor device is reduced so that the crystal substrate can be protected from a crack.
It is noted that in the sensor device according to the present invention, a Langasite crystal having a large mechanical coupling coefficient may be used in place of the crystal substrate.